The invention relates to a brake pressure transducer (brake pressure generator) for a hydraulic vehicle brake system. Furthermore, the invention relates to a hydraulic vehicle brake system that is equipped with such a brake pressure transducer and also to a method of operating such a brake pressure transducer and a vehicle brake system equipped therewith.
It is generally known that, in hydraulic vehicle brake systems, the brake pressure transducer comprises a so-called main brake cylinder in order to generate a brake pressure for the wheel brake that is proportional to the actuating force initiated via the actuating elementxe2x80x94normally a brake pedal. It is likewise generally known also to equip the brake pressure transducer with a brake force booster that superimposes a servo force to boost the actuating force initiated via the actuating element. Suitable as brake force boosters for this purpose are both pneumatic boosters, which operate on the underpressure principle, and hydraulic boosters, which employ a hydraulic pump.
Such a pneumatic brake force booster is disclosed, for example, in DE 28 45 794 C2, while such a hydraulic-brake force booster is disclosed, for example, in DE 44 43 869 A1. Both the pneumatic and the hydraulic brake force boosters have a movable partition that subdivides an internal housing space into two chambers and transmits a force via a transmission element to the main brake cylinder if the chambers are subjected to a pressure difference as a function of a force acting on the actuating element. In the unactuated state, the chambers are pressure-equalized, with the result that the movable partition does not transmit a force to the output member. In the case of the pneumatic booster, the pressure difference is produced by an underpressure being generated in one chamber by means of an underpressure source, while the other chamber is subject to atmospheric pressure. In contrast, in the hydraulic booster, the pressure difference is generated by means of a hydraulic pump whose suction side is connected to the one chamber and whose pressure side is connected to the other chamber, with the result that the hydraulic pump pumps in the direction from the one chamber to the other chamber in order to achieve a brake force boost.
Nevertheless, such a hydraulic vehicle brake system is open to improvement. Thus, the full brake force boost is needed only in about 10% of all braking actions relating to a vehicle. It is now clear that the design of the brake force boost is overdimensioned for the remaining about 90% of the braking actions. This overdimensioning has the disadvantage that a relatively large installation space is necessary in the motor vehicle, as a result of which complexity and costs occur.
Especially if a pneumatic brake force booster is used, there is a direct relationship between booster power and overall size, that is to say the greater the booster power required, the larger is the brake force booster. Since the required booster power depends substantially on vehicle weight, so-called tandem boostersxe2x80x94that is to say, in principle, two brake force boosters arranged behind one anotherxe2x80x94have to be predominantly used in higher vehicle classes, as a result of which further installation space is needed in addition. A pneumatic brake force booster furthermore has the disadvantage that an underpressure source has to be provided in the vehicle. True, in the case of a vehicle equipped with a petrol engine, the underpressure generated in the intake system can in principle be used. However, severe fluctuations in the underpressure generated in this way can adversely affect the performance of the brake system, in particular with regard to the ever-increasing performance requirements in the future, so that complexity and costs of providing an independent underpressure source are unavoidable.
The object of the invention is therefore to provide an improved hydraulic vehicle brake system that can be produced in more compact form and with a lower cost expenditure and can also be actuated comfortably.
This object is achieved according to the invention with a brake pressure transducer that has the features disclosed herein.
According to the invention, the quantity of brake fluid that accrues because of the reduction in the volume of the additional hydraulic chamber at the input side of the pump is additionally fed, when the brake pressure transducer is actuated, by means of the pump to the quantity of brake fluid accruing in the wheel brake because of the reduction in the volume of the hydraulic chamber. The said quantity of brake fluid additionally fed into the wheel brake by means of the pump has the effect that a higher brake pressure than the brake pressure originally generated in the hydraulic chamber is established in the wheel brake or in the hydraulic chamber. In this way, a secondary servo force is provided by means of which a boost in the actuating force initiated via the actuating element and/or the primary servo force provided by the brake force booster can be achieved in a particularly advantageous way. The total actuating force of the brake pressure transducer is consequently made up of the initiated actuating force, the primary and the secondary servo force.
Under these circumstances, the disadvantages explained above in the case of the use of a generally known brake force booster are avoided since the brake force booster providing the primary servo force has to apply a substantially lower boost. In the case of a pneumatic brake force booster, this means specifically that a single booster having a diameter of 6 inches is adequate for a vehicle that, in the case of a conventional brake force system, would have to be equipped at least with one tandem booster having a diameter of 8 and 9 inches. In addition, because a substantially xe2x80x9cweakerxe2x80x9d pneumatic brake force booster is adequate, the underpressure source can also be xe2x80x9cweakerxe2x80x9d and to that extent can be designed more simply and with a lower cost expenditure. Since a xe2x80x9cweakerxe2x80x9d pneumatic brake force booster is also less sensitive to fluctuations in the underpressure, the underpressure generated by reason of principle in the intake system can be utilized in a less critical way in the case of a vehicle equipped with a petrol engine.
A further substantial advantage is that two servo forces are applied that are independent of one another. This consequently comprises a redundancy should a failure occur either of the brake force booster providing the primary servo force or the pump providing the secondary servo force, as a result of which a decisive contribution is made to increasing the safety of the vehicle brake system.
The brake force booster may be a brake force booster in the conventional sense that superimposes a fixed primary servo force on the actuating force initiated via the actuating element. On the other hand, it may be an electronically controllable brake force booster that can be controlled by means of an electrical actuator in order, firstly, to actuate the brake pressure transducer instead of or in addition to an actuation via the actuating element and, secondly, to adjust the primary servo force. Suitable as an electrical actuator is, preferably, a solenoid valve arrangement that is incorporated in the electronically controllable brake force booster in an installation space-saving manner. As a result of the use of an electronically controllable brake force booster, the vehicle brake system becomes particularly suitable for emergency or spot braking actions and also automatic braking procedures, for example, for regulating vehicle dynamics, drive slip and distance.
In an advantageous manner, the regulating behaviour of the vehicle brake system with regard to actuating comfort, which means reactions on the actuating element (brake pedal), and metering capability are improved if the delivery rate of the pump can be controlled by means of an electrical actuator in order to adjust the secondary servo force. To be preferred here as an electrical actuator is an electric motor whose rotational speed can be regulated in order to adjust the delivery rate.
Furthermore, a valve device can be provided through which, in a first position, a fluid connection exists between the output of the additional hydraulic chamber and the output of the hydraulic chamber only via the pump and, in a second position, a fluid connection exists directly between the output of the additional hydraulic chamber and the output of the hydraulic chamber. If the valve device is in its second position, the pump is as it were shunted, with the result that no secondary servo force is provided. Consequently, the actuating force initiated via the actuating element is boosted only by the primary servo force provided by the brake force booster, and, as already mentioned, this is adequate for about 90% of vehicle braking actions. Consequently, the pump could be driven continuously, for example, by an operative coupling of the pump with drive unit, present in any case in a vehicle, simply existing by means of a drive belt. Only if one of the full vehicle braking actions having a proportion of about 10% already mentioned has to be performed, does the valve device assume its first position so that the secondary servo force is additionally provided via the pump. For this purpose, the valve device is preferably electromagnetically actuable, in which connection it assumes its first position as an actuating position and its second position as its basic position under spring actuation.
Since vehicle brake systems normally have two separate brake circuits, provision is made that there is connected in series with the hydraulic chamber a second hydraulic chamber whose volume likewise decreases when the brake pressure transducer is operated in order to generate a brake pressure for at least one further wheel brake. Consequently, the two hydraulic chambers can each generate in a braking circuit assigned to them a brake pressure for the respective wheel brakes, regardless of whether, for example, a diagonal partitioning or a front/rear partitioning is provided for the vehicle.
So that the same brake pressure is established in the brake circuits, the brake pressure transducer is designed so that, when the brake pressure transducer is actuated, the volume of the second hydraulic chamber decreases to the same extent as the volume of the hydraulic chamber. Ideally, the second hydraulic chamber is formed by a floating piston disposed in an axially sealing and displaceable manner in a common bore of the brake pressure transducer separating the two hydraulic chambers from one another.
So that a secondary servo force of the order of magnitude of the primary servo force provided by the brake force booster can be achieved, the brake pressure transducer is dimensioned so that, when the brake pressure transducer is actuated, the volume of the hydraulic chamber decreases to a percentagewise lesser extent than the volume of the additional hydraulic chamber.
To control the brake pressure transducer, an electronic control unit is provided that determines at least one variable relating to the actuation of the brake pressure transducer by means of sensors in order to activate the electrical actuators as a function thereof. In this connection, the variable(s) relating to the actuation of the brake pressure transducer may, for example, be the brake light switch signal, the distance initiated at the actuating element, the force initiated at the actuating element, the speed with which the actuating element is actuated, the pressure difference accruing in the brake force booster, the pressure generated in the hydraulic chamber and variables derived therefrom. The actuators, if they are electrically drivable, may be, for example, the electric motor driving the pump, the valve device shunting the pump and the solenoid valve arrangement controlling the brake force booster. This makes it possible in the simplest application case for the pump to be actuated only if the actuating element is actuated, with the result that drive energy is reduced and permanently occurring drive noises are avoided.
A decisive contribution is made to reducing components and consequently costs if an anti-lock/drive-slip regulating device is disposed between the brake pressure transducer and the wheel brake, the pump being a component of the anti-lock/drive-slip regulating device. As a result, the secondary servo force is provided by means of the pump present in any case in the anti-lock/drive-slip regulating device. Furthermore, this results in the advantage with regard to reduction of installation space that the brake pressure transducer and the anti-lock/drive-slip regulating device can be integrated to form a compact assembly. Furthermore, as a result of the integration, the connecting lines otherwise necessary between the brake pressure transducer and the anti-lock/drive-slip regulating device are unnecessary, as a result of which the risk of leakages is minimized and system safety is gained.
In this connection, there is provided for the anti-lock/drive-slip regulating device an electronic control unit that determines at least one variable relating to the dynamic behaviour of the vehicle by means of sensors in order to control, as a function thereof, the brake pressure in the at least one wheel brake by means of electrical actuators. The variable(s) relating to the dynamic behaviour of the vehicle is/are, for example, the wheel or vehicle speed(s), wheel or vehicle deceleration(s) and reference values derived therefrom; if the vehicle brake system is also designed for vehicle dynamics regulation, the variable(s) is/are the longitudinal or transverse acceleration and the steering angle of the vehicle and, if the vehicle brake system is designed for distance regulation, the distance from an obstacle.
There is therefore an advantage if the electronic control unit of the brake pressure transducer and the electronic control unit of the anti-lock/drive-slip regulating device communicate with one another via data lines or a common electronic control unit is provided for the brake pressure transducer and the. As a result, operation of the brake pressure transducer is possible as a function of the operating state of the anti-lock/drive-slip regulating device and vice versa. In particular, however, the electrical actuators of the brake pressure transducer can be controlled as a function of the variable(s) relating to the dynamic behaviour of the vehicle, and equally, the electrical actuators of the anti-lock/drive-slip regulating device can be controlled as a function of the variable(s) that relate to the actuation of the brake pressure transducer and that represent, inter alia, the braking requirement of the vehicle driver. If the electronic control units of the brake pressure transducer and of the anti-lock/drive-slip regulating device are designed as separate units, a bus system present in any case in the vehicle, such as, for example, a CAN bus, can advantageously be concomitantly used as data line for the communication.
A particularly advantageous method of operation consists in that, if the gradient of brake pressure to total actuating force is determined by the initiated actuating force and/or the primary servo force and the gradient of brake pressure to total actuating force drops below a preset gradient, the secondary servo force is superimposed on the initiated actuating force and/or the primary servo force to such an extent that the preset gradient is at least maintained. This results in a very economical mode of operation, in particular if the gradient of brake pressure to total actuating force is determined by the initiated actuating force and the primary servo force, as in the case of a conventional brake pressure transducer. Consequently, the secondary servo force has to be additionally applied by the pump only if the primary servo force provided by the brake force booster has been applied, that is to say the run-out point of the brake force booster compared with a conventional brake force transducer has been reached. The boost characteristic is therefore extended beyond the run-out point by means of the secondary servo force provided by the pump from the time when the run-out point is reached. An application example of this is formed by the vehicle braking actions already mentioned at the outset having a proportion of 10% for which a high (full) braking force boost is needed.
Furthermore, it is particularly advantageous for the operation to proceed in such a way that, if the gradient of brake pressure to total actuating force is determined by the initiated actuating force and/or the primary servo force and is equal to a preset gradient and at least one variable relating to the actuation of the brake pressure transducer differs from a preset value and/or at least one variable relating to the dynamic behaviour of the vehicle deviates from a preset value, the secondary servo force is superimposed on the initiated actuating force and/or the primary servo force to such an extent that the gradient of brake pressure to total actuating force is equal to a further preset gradient that is greater than the preset gradient. If the gradient of brake pressure to total actuating force is determined by the initiated actuating force and the primary servo force, as in the case of a conventional brake pressure transducer, the secondary servo force is consequently additionally applied in this case compared to a conventional brake pressure transducer already before the run-out point of the brake force booster is reached. A switchover to a steeper boost characteristic thus takes place. An application example of this is an emergency or spot braking action that is performed if the driver""s requirement that results from the variable(s) relating to the actuation of the brake pressure transducer demands a higher vehicle deceleration than the actual vehicle deceleration that results from the evaluation of the variable(s) relating to the dynamic behaviour of the vehicle.
So that the full boost is applied by the brake pressure transducer, the preset gradient is equal to the gradient that results at maximum primary servo force. Furthermore, the further preset gradient corresponds to the gradient that results at maximum primary servo force and maximum secondary servo force.